In recent years, with diversification of services provided by radio communication, there is a demand for multiband operation in radio equipment capable of dealing with information in plural frequency bands. For example, two frequency bands, which are 5.2 GHz and 2.4 GHz bands, are specified in the respective standards of Institute of Electrical and Electronic Engineers (IEEE) 802.11a/b/g that are the technical standards of wireless Local Area Network (LAN).
As one of the devices implemented in the radio equipment, a power amplifier is used that amplifies signals of a radio frequency band and supplies the signals to an antenna. A power amplifier is one of such devices that consume a large amount of power in a radio circuit, and are required to operate at a high efficiency. Generally, in the design of a radio circuit, optimization has been made for only a specific frequency band. In the design of a power amplifier, for example, the optimization includes obtaining a high output power and a high efficiency. It is therefore difficult to design such a circuit that is optimized for both of the aforementioned two different frequency bands, for example. For this reason, it is generally configured such that a switch selectively changes the circuits optimally designed for respective frequency bands.
FIG. 1 illustrates an example of the circuit configuration generally employed for a power amplifier (dual-band power amplifier) capable of amplifying signals of two frequency bands. For example, it is assumed that the center frequencies of the two frequency bands are set, such as f1=5.2 GHz and f2=2.4 GHz. A dual-band power amplifier 900, as illustrated in FIG. 1, is provided with: an amplifier 921 designed exclusively for the frequency band of the center frequency f1 (hereinafter, simply referred to as frequency band of the frequency f1 or frequency band of f1, the same applies to f2); and an amplifier 922 designed exclusively for the frequency band of the center frequency f2. Either the amplifier 921 or the amplifier 922 is chosen by switching a single-pole double-throw (SPDT) switch 911 connected to an input terminal 931 and an SPDT switch 912 connected to an output terminal 932, according to the operating frequency, namely the frequency f1 or f2.
Koji Chiba, Isao Hirakodama, Toru Takahashi, Naoki Naruse and Hisashi Yoshinaga, “Mobile Terminals” NTT DoCoMo Technical Journal, Vol. 14, No. 1, for example, discloses such a conventional technique.
Each of the amplifiers 921 and 922 of FIG. 1 includes: an input-side matching circuit 971; an amplification device 972; and an output-side matching circuit 973, as depicted in FIG. 2. The performance of the amplifier depends on the characteristics of the amplification device and those of the matching circuits. It is therefore important to optimize the matching circuits at the frequency bands at which the amplifier operates. The circuit configuration of the dual-band power amplifier 900 of FIG. 1 allows each amplifier to use the matching circuit optimized for the frequency band only for the amplifier. The two amplifiers each provided with such optimized matching circuits are switched by the SPDT switches according to the operating frequency band. Hence, if the insertion loss of the SPDT switch is sufficiently small, the amplifier with the characteristics of high output and high efficiency will be available, accordingly.
Such a dual-band power amplifier requires two systems of circuits, in total, which includes an amplifier for the frequency f1 and an amplifier for the frequency f2. This causes a problem that the number of components such as the input and output matching circuits, amplification devices, etc will be increased. The increased number of the components also causes other problems that the device size is increased, and in addition, the power consumption in the whole circuit is increased by the power consumption in each component.
In addition, there is another problem that the output power is lowered by the insertion loss of the SPDT switch used for output in particular, thereby decreasing the efficiency.
Furthermore, when a combined signal of two frequency bands is simultaneously amplified with a high efficiency in each of the frequency bands, it is necessary to employ a splitter and a combiner for the SPDT switches 911 and 912, respectively. This has a drawback of increasing the circuit size (see Japanese Patent Application Laid Open No. 2003-504929, for example).
Thus, there exists a need for a dual-band power amplifier capable of amplifying a combined signal of two frequency bands at each of the frequency bands simultaneously without providing two systems of circuits for the amplifier for f1 and the amplifier for f2. There also exists a need for a matching circuit capable of performing the impedance matching on the combined signal of the two frequency bands, suited for such a dual-band power amplifier, at each of the frequency bands simultaneously. It should be appreciated that since such a matching circuit can be broadly used for a device that has to perform impedance matching between the signals of two frequency bands, such a matching circuit is not limited to be used in the dual-band power amplifier.